The Status field in the Purchase Header Table indicates whether the Purchase Header is ready for next stage of processing or not. The are options in the Status field are Open and Released. If the Status is Open, the Purchase Header can be changed. If the Status has been changed to Released, the Purchase Header cannot be changed anymore.
After the status has been released and you still want to make changes to the document, you can re-open it. You can change the status by Clicking on Function --> Reopen or press Ctrl + F11. After make the changes, you can change the status back to Released by clicking on Function --> Release.
This field will automatically change to Released when you do a Post Receive.